Overeating, excessive weight gain and increases in body fat content occur when adult rats of either sex are offered a nutritionally complete diet and a palatable carbohydrate solution. These effects are observed with diets that range in protein content from 10 percent - 40 percent and with glucose, maltose and sucrose solutions. Overeating is eliminated in male rats housed in an activity wheel or with a sexually receptive female. Overeating is also eliminated if access to either dietary component (complete diet or carbohydrate solution) is constrained experimentally. These data appear to have many parallels to the conditions associated with adult-onset obesity in humans. The objectives of the proposed research are to study the behavioral and physiological dimensions of obesity in these animals and to specify the mechanisms that control this form of obesity. Studies on the behavioral antecedents and consequences of dietary obesity will study feeding patterns, defense of body weight, taste responsivity and the effects of social housing conditions. Body composition, pancreatic morphology and oxygen consumption will be measured to characterize the obese animals physiologically. One series of studies concerned with the mechanisms underlying this form of obesity will try to distinguish between dietary diversity and palatability as the critical dietary dimension that encourages overeating. Another series of studies concerned with the controlling mechanisms will examine dietary selection patterns when graded amounts of carbohydrate solutions or a complete diet are delivered intragastrically.